I personally think this is a fun idea. Of course you can drink just water, like many point out, but the reason to vary the intake types is to overcome flavor boredom. Very common phenomenon. It’s why it’s easier to eat a lot of many kinds of foods than a lot of one food only (ie the foundation of many funny restriction “diets”). This tip takes advantage of that psychological phenomenon and uses it to make a healthy habit fun! It’s the same reason I have many kinds of tea and seltzer flavorings.